Process and apparatus for heat exchange

ABSTRACT

A process and apparatus is provided for storing thermal energy and subsequently releasing and extracting the stored thermal energy upon demand. At least one sealed container of salt hydrate is agitated continually and is positioned in heat exchange relationship with a heat exchange liquid which is passed between a thermal energy source and a container enclosing or partially enclosing the sealed container(s) for the salt hydrate. Agitation of the container(s) of salt hydrate prevents or minimizes salt separation and supercooling so that the latent heat of fusion of the salt hydrate can be stored and extracted by the heat exchange liquid upon demand, in addition to the sensible heat of the salt hydrate composition.

This is a division, of application Ser. No. 735,418 filed Oct. 26, 1976now U.S. Pat. No. 4,117,882.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for storing andsubsequently releasing thermal energy. More particularly, this inventionrelates to a method and apparatus for storing and subsequently releasingthermal solar energy.

Prior to the present invention, a wide variety of salt hydrates havebeen utilized to store heat for subsequent extraction upon demand. Thesesalt hydrates are useful for this purpose since each is characterized bya relatively high heat of fusion but the phase change between solid andliquid in each occurs at a different temperature within a moderate rangeof temperatures. Heat can be stored both as sensible heat and as thelatent heat of fusion of the selected salt hydrate for subsequentrelease during crystallization of that salt hydrate at its usualphase-change temperature by heat exchange with any of a variety of heatexchange liquids.

Significant problems associated with all the salt hydrates have greatlylimited their use as heat storage media. Many salt hydrates are prone tosuper-cooling so that the phase change from liquid to solid does notreadily occur and the latent heat of fusion is not recovered whendesired. This phenomenon has necessitated adding nucleating agents tothe salt hydrates to minimize supercooling. Even the presence ofnucleating agents does not assure that the salt hydrate will crystallizeupon cooling. The salt hydrates also have a tendency to becomedehydrated gradually when exposed to repeated phase-change thermalcycling. Dehydration results in the salts developing different densitiesdependent upon the degree of hydration, with accompanying separation andstratification of the salts. When this separation occurs, it becomesincreasingly difficult to cause the salts to undergo phase changeconcurrently since they have developed different melting points. Thus,some of the salts in a container will not undergo a phase change and maynot release the latent heat of fusion that otherwise would have beenextracted during a given thermal cycle. It has been proposed also tosuspend the salts in gelatinous types of medium to overcome theseparation problem. However, this reduces the thermal capacity of theresultant composition on a volume basis and thereby reduces its heatexchange effectiveness.

It has also been proposed to agitate a container of the salt hydrate toprevent or minimize supercooling at or near the temperatures at whichthe salt hydrate undergoes phase change. In these methods, a heatexchange fluid is passed continuously into heat exchange relationshipwith the agitated container and then either is directed back to thesource of heat or is directed to the area of ultimate use. In theseproposals, the salt hydrate is the sole means for thermal storage whilethe heat exchange fluid is used solely to carry heat to the area ofthermal demand or to transfer heat from the thermal energy source to thesalt hydrate and this requires relatively high mass flow rates of heatexchange fluid, effective heat exchange surfaces and large amounts ofsalt hydrate.

Accordingly, it would be desirable to provide a means for storingthermal energy based upon the use of salt hydrates which avoids theproblem of supercooling, which minimizes or prevents salt separation andmodification caused by repeated thermal cycling and which provides lowcost, effective heat exchange between the salt hydrate and the heatexchange fluid.

SUMMARY OF THE INVENTION

In accordance with this invention, one or more salt hydrates are storedin one or more closed first containers capable of being agitated andwhich are enveloped or partially enveloped by a second containercontaining a heat exchange liquid capable of extracting thermal energyfrom the salt hydrate and capable of transmitting thermal energy to thesalt hydrate. The salt hydrate container(s) are sealed and the containerfor the heat exchange liquid is provided with inlets and outlets,through which the heat exchange liquid is moved. The volume ratio ofheat exchange liquid in the second container to the salt hydrate isbetween about 1 to 20 and 2 to 1, preferably between 1 to 10 and 2 to 1.

In operation the heat exchange liquid is exposed to any source ofthermal energy, including solar radiation. The heated liquid is passedinto the second container to transmit thermal energy to the salt hydrateand then is passed out from the second container either to means ofextracting energy from the heat exchange liquid or to redirect theliquid to the source of thermal energy. The salt hydrate obtains thermalenergy from the heat exchange liquid is sensible and/or latent heat sothat, at a later time, the stored thermal energy can be re-transmittedto the heat exchange liquid for ultimate use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of a thermal storage apparatusof this invention.

FIG. 2 is a cross-sectional view of the apparatus of FIG. 1 taken alongline 2--2.

FIG. 3 is an alternative embodiment wherein heat exchange liquid ishoused in a jacket surrounding a sealed container of salt hydrate.

FIG. 4 is an alternative embodiment of this invention utilizing aplurality of sealed containers for the salt hydrate composition orcompositions.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to FIGS. 1 and 2, a container 10 houses a salt hydrate havinga heat of fusion greater than about 50 BTU/lb, preferably greater than75 BTU/lb. The container is supported on idle rollers 22 and movablerollers 12 supported on shaft 14 which is rotatably mounted on walls 16and 18 of container 20. Idle rollers 22 are mounted also on a shaft (notshown) which is also rotatably mounted on walls 16 and 18. The rollers12 and 22 are mounted at or near the ends of container 10 so thatdeflection of the walls of the container 10 is minimized. Shaft 10 androllers 22 are activated by motor 24 attached to shaft 14.

Container 20 houses a heat exchange liquid 26 which surrounds container10. The heat exchange liquid receives thermal energy in heat exchangeunit 28 wherein the source of heat is not critical. For example, theheat source can be any means of combustion or direct or indirect solarenergy radiation or electric energy. In relation to the heat exchangeunit 28, there can be solar panels of any design which receive and trapradiant energy and which are in heat exchange relationships with theheat exchange liquid in the serpentine conduits 30. The heated liquid ispassed from conduit 30 to conduit 32, through inlet 34, into container20. The heated liquid in container 20 passes its heat through thecontainer wall 10 to the salt hydrate in the container 10 therebyliquefying the salt hydrate so that it acquires both latent heat andsensible heat which later can be extracted upon demand.

Heat exchange liquid is circulated through the system by pump 36. Valve38 is positioned to regulate liquid flow through conduit 40 to recycleliquid to the heat exchanger 28 and/or through conduit 42 to extractheat for use such as in a home heating system. Liquid from which heathas been extracted is returned to the system by conduit 44 when valve 46is open.

For a given weight of salt hydrate, the volume of salt hydrate affords ameasurement of the quantity of latent heat contained therein. The volumeof salt hydrate increases with the increase of latent heat stored sincea greater proportion of the salt hydrate is converted to liquid. Forexample, the volume of sodium thiosulfate pentahydrate increases about9% when changing from total solid to total liquid. Accordingly, apressure gauge 48 positioned to communicate with the container interior,preferably on its axis of motion, provides an analog measurement of thelatent heat in the salt hydrate. Thermostats (not shown) are positionedin or adjacent conduits 30 and in the heat exchange liquid 26 to sensethe temperature of the heat exchange liquid. The thermostats are linkedin any conventional manner (as represented by the broken lines) tocontrol pump 36 and valves 38 and 46 such as by pneumatic or electricallinkage. In addition, valves 38 and 46 and pump 36 can be linkedelectrically or pneumatically to conventional control means such as athermostat in the area of use to control heat exchange liquid flow intothe area of use.

The pressure gauge 48 provides a means by which the user can determinethe latent heat content of the salt hydrate at any given time. Thus, theuser can read gauge 48 and initiate the auxiliary heater 49 (e.g.electrical heater) to heat the heat exchange liquid 26. Alternatively,the user, upon noting that the salt hydrate contains less stored heatthen desired, can manually override the automatic system to initiatepump 36 to circulate heat exchange liquid between the heat exchange unit28 and the container 20. This quick and accurate determination of heatcontent in the salt hydrate is particularly useful in climates whichexperience seasonal changes when the heat exchange unit 28 is a solarheat exchange unit. In the winter, the auxiliary heater 49 and/orcirculation between the solar heat exchange unit and the container 20could be initiated to keep the salt hydrate at a relatively highpressure reading, while in summer such activation could be related to adifferent threshold pressure reading.

The container 10 is continuously rotated or oscillated by motor 24 androllers 22 and is provided internally with mixing arms 50 extending fromthe wall of container 10 so that the salt hydrate 52 is maintained in anagitated condition. Continuous agitation of the container and of thesalt hydrate 52 minimizes or prevents salt hydrate supercooling andminimizes or prevents stratification and dehydration of salt hydrate.

Referring to FIG. 3, a container 11 is partially filled with the salthydrate and is surrounded by a jacket 13. The jacket is sealed to theouter surface of container 11 and is filled with heat exchange liquid.The jacket 13 is provided with an inlet conduit 15 and an outlet conduit17. The container is mounted on powered rollers 19 and idle rollers (notshown) such as in the manner shown in FIG. 2. The powered rollers aremounted on shaft 21, attached to motor 25 and rotatably attached tosupport 23. Since, in this mode, the container 13 for the heat exchangeliquid is movable, the conduits 15 and 17 are flexibly connected to thejacket 13 to minimize mechanical stress in the conduits caused byagitating the container 11 and jacket 13. In addition, the container 11is agitated in an oscillating mode so that mechanical stress on theconduits 15 and 17 is minimized and so that the conduits do not contactthe rollers supporting the container 11.

Referring to FIG. 4, a plurality of sealed containers 10 that contain asalt hydrate composition or compositions are immersed in a heat exchangeliquid 26 which is enclosed in container 20 provided with an inlet 34and an outlet 35. The containers are mounted on struts 37 which are, inturn, mounted on rotating shaft 39. This embodiment provides enlargedheat exchange surface area between the heat exchange liquid 26 and thesalt hydrate sealed within containers 10. In this embodiment, differentdrums can contain different salt hydrates so that the latent heat offusion could be extracted or stored over a wider temperature range thanis available when each container contains the same salt hydratecomposition.

An important aspect of this invention is the combined heat storage andrelease capacities of the heat exchange liquid in the container 30 orjacket 13 and the salt hydrate in container 10. The volume of heatexchange liquid in such container or jacket should be about between 1 to20 and 2 to 1, preferably between about 1 to 10 and 2 to 1 volume ratioto the salt hydrate volume in its container. Thus, when employing wateras the heat exchange liquid and sodium thiosulfate pentahydrate as thesalt hydrate, the ratio of water volume to salt hydrate volume should bebetween about 1 to 10 and 2 to 1. At volume ratios higher than abovestated, the temperature change in the water may be too small to be ofpractical use. At volume ratios lower than above stated, the lesserquantity of water does not provide the desired heat buffering capacity.

Representative useful salt hydrates in the present invention includesodium thiosulfate pentahydrates, calcium chloride hexahydrate, sodiumcarbonate decahydrate, disodium phosphate dodecahydrate, calcium nitratetetrahydrate, sodium sulfate decahydrate or mixtures thereof or thelike. Although not essential, the salt hydrate optionally can include anucleating agent such as borax or the like in concentrations generallybetween about 0.5 wt % and 5 wt. % based upon the weight of the salthydrate. In addition, the salt hydrate can include an anticorrosionagent to prevent corrosion of the salt hydrate container. Representativesuitable anticorrosion agents include sodium dichromate or the like. Theanticorrosion agent is not essential when employing a container madefrom a corrosion-resistant material such as stainless steel or heavygauge cold rolled steel. In one aspect of this invention, water can beadded to the salt hydrate for a variety of purposes, including but notlimited to (1) promoting slurry-like crystallization in multiplicity ofsmall crystals, (2) lowering the salt hydrate freezing point so as torelate the heat of fusion temperature during winter months to thetemperature range of the heat source(s) and of the heat exchange liquidin winter. In summer months, the added water can be removed to raise thefreezing point of the salt hydrate composition in relation to thetemperature range of the heat exchange liquid and its principal heatsource. Representative suitable heat exchange liquids include water,Dow-therm, water plus ethylene glycol of the like. In addition, thewater also is useful to render the salt hydrate crystaline mass lesshard and tenacious to the wall of the container.

In a representative embodiment of this invention, the apparatus of FIGS.1 and 2 can be formed of a sealed stainless steel cylinder, 4 feet indiameter and 9 feet long which is 85% to 95% full with sodiumthiosulfate pentahydrate having a phase change temperature of 120° F.The container housing the steel cylinder is formed of wood and isrectangular shaped having 10 ft.×5 ft.×5 ft. dimensions provided with awaterproof liner. The wooden container houses about 3 tons of water. Themotor is rated at 1/15 HP to effect rotation of the steel cylinder at 4revolutions per hour on a continuous basis. At this rate, the motor usesonly about 2 Kw/hr of energy per daytime period in winter. In operation,water is circulated through the wooden container from a source of heatto provide indirect exchange of heat to the sodium thiosulfatepentahydrate. The salt hydrate has a volume of about 104 cubic feet, adensity of about 104 lb/cubic foot, a total mass of about 11,000 poundsand a latent heat of phase exchange of about 90 BTU/lb. The total amountof heat liberated when the salt changes from liquid to solid is about990,000 BTU which is enough heat to maintain a moderate size,well-insulated house in the northeastern United States warm for two daysin a typical sunless period in January.

Continual rotation or oscillation of the steel container prevents orsharply minimizes formation of stagnant layers at the inside surface andat the outside surface of the steel cylinder. This both improves heatexchange and greatly extends the number of effective salt hydrate heatcyclings. The water in the wooden container responds more quickly thanthe salt hydrate; this assists heat transfer to and from the salthydrate. For example, the 3 tons of water mentioned could absorb asudden pulse of 100,000 BTU, and go on transferring it to the salthydrate which has a slower rate of response. Likewise, 100,000 BTU ofheat could quickly be supplied from the water, with further heatextraction from the salt hydrate into the water to maintain its supplyof heat to points of ultimate use.

It is to be understood that variations can be made from the specificembodiments described above. For example, the container for the salthydrate can be filled only 55 to 60% so that it will float and thefloating drum can be rotated with a perimeter friction-drive motorattached to the housing for the heat exchange liquid. Also, thecontainer for the salt hydrate can include a self-cleaning device suchas sliding metal disks to clean its inner wall of solid salt hydrate. Inaddition, the container for the salt hydrate and/or the container forthe heat exchange liquid can be provided with an auxiliary electricalheating element to provide off-peak energy to the salt hydrate.

I claim:
 1. A process for storing thermal energy for subsequent usewhich comprises:exposing a heat exchange liquid to heat to increase thethermal energy in said liquid, passing the heated liquid into a firstcontainer in heat exchange relationship with a composition including asalt hydrate enclosed in at least one sealed container, said salthydrate having a heat of fusion of at least about 50 BTU/lb, the volumeratio of the heat exchange liquid in said first container to salthydrate being between about 1 to 20 and 2 to
 1. each sealed containerbeing continually agitated to prevent or minimize supercooling andirreversible separation of said salt hydrate, monitoring the pressurewithin at least one of said sealed containers thereby to determine thelatent heat stored in the salt hydrate within said containers, andperiodically passing said heat exchange liquid from indirect heatexchange relationship with said salt hydrate to extract heat from saidliquid.
 2. The process of claim 1 wherein agitation of said container isconducted by rotation.
 3. The process of claim 1 wherein agitation ofsaid container is conducted by oscillation.
 4. The process of claim 1wherein said container is immersed in said heat exchange liquid housedin a second container.
 5. The process of claim 2 wherein said containeris immersed in said heat exchange liquid housed in a second container.6. The process of claim 4 wherein said container is partially filledwith said salt hydrate composition to effect suspension of saidcontainer in said heat exchange liquid.
 7. The process of claim 1wherein the salt hydrate is sealed in a plurality of containers andwherein said means for monitoring pressure is associated with at leastone of said sealed containers.
 8. The process of claim 1 wherein saidheat exchange liquid is recycled from said first container to a meansfor exposing said heat exchange liquid to the step of exposing the heatexchange liquid to heat thereby to increase the thermal energy in saidliquid.
 9. The process of claim 8 wherein said heat exchange liquid isheated by solar heat.
 10. The process of claim 1 wherein the heatexchange liquid is heated to increase its thermal energy by beingexposed to a heat source adapted to transfer solar heat to said liquid.11. The process of claim 1 wherein said heat exchange liquid in saidfirst container is heated in response to the pressure monitored withinsaid sealed container.
 12. The process of claim 7 wherein at least oneof said sealed containers contains a salt hydrate different from a salthydrate in at least one of said remaining sealed containers.